The broad goal of this project is to investigate the process of right ventricular remodeling in patients with pulmonary hypertension before and after therapy using magnetic resonance imaging. The following hypothesis will be tested: The MRI-derived indices of global, regional, and hemodynamic measures of RV performance will correlate more closely with the patient's clinical course and better predict outcome than the standard methods of assessment (clinical examination and serial echocardiography.) The time course of remodeling will be dependent on the afterload reduction achieved and the etiology of the increased afterload.